1. Field of the Invention
The present invention generally relates to gas pressurized hydraulic dampers, and more particularly, to a method and apparatus for charging such dampers with pressurized gas.
2. Description of the Related Art
Gas pressurized hydraulic dampers, such as shock absorbers and struts, are well-known and have found numerous applications on road driven vehicles and most notably on automobiles and light trucks. Such gas pressurized hydraulic dampers generally have an inner pressure cylinder and outer reserve cylinder with the space therebetween forming a reserve chamber which is used to store a pressurized gas such as nitrogen. The pressurized gas which is located within the reserve chamber is used for improving ride comfort and road holding ability.
Several methods have been developed for introducing the pressurized gas into the reserve chamber of the damper during manufacture. In this regard, U. S. Pat. No. 4,564,138, commonly owned by the assignee of the present invention and the disclosure of which is expressly incorporated by reference, discloses a method for charging a shock absorber with pressurized gas. The method involves forming a hole in the reserve chamber of the shock absorber cylinder after completing assembly of the shock absorber. Pressurized gas is allowed to flow through the hole and into the shock absorber. The hole is then sealed by placing a metal ball over the hole and welding the ball to the cylinder.
Other methods, such as those disclosed in U.S. Pat. Nos. 4,491,160 and 4,131,138, provide for charging a shock absorber with pressurized gas by forming a locally reduced cross-sectional area on the piston rod. The reduced cross-sectional area is aligned with the seal to create a passage for pressurized gas into the shock absorber. Once the pressure of the gas within the shock absorber has reached the desired level, the rod is displaced axially relative to the seal so that the full cross-sectional area of the piston rod engages the seal so as to retain the gas within the shock absorber.
A further method for charging a damper with pressurized gas is disclosed in U.S. Pat. No. 4,044,866. The method disclosed in this reference also incorporates use of a reduced cross-sectional area formed on the piston rod which is able to be aligned with a seal to form a passage for charging the damper with pressurized gas. After the damper is charged, the seal is displaced to prevent reformation of the passage during use and thus leaking of the gas from the damper.
Another method of charging a damper with gas is disclosed in U.S. Pat. No. 4,114,866. The method disclosed in this reference involves axially displacing the seal away from an upper portion of the cylinder assembly of the damper and against the resistance of a retaining spring. Pressurized gas is then passed between the displaced seal and the cylinder. Once the desired level of pressure is obtained, the force displacing the seal is released, and the seal moves into contact with the cylinder assembly under urging of the retaining spring thus sealing the gas within the damper.
Still another method of charging a damper with gas is commonly known as auto-nibbing. Auto-nibbing involves inserting a nib between the piston rod seal and the piston rod and injecting pressurized gas therethrough into the shock absorber.
While these methods are effective, they also suffered from several disadvantages. For example, methods involving the use of a piston rod with a reduced cross-sectional generally require additional processing to form the reduced cross-sectional area on the piston rod. In addition, the piston rod must be precisely aligned with the seal to form the passage for the gas during the charging operation. Furthermore, some provision must be made for preventing the piston rod from realigning with the seal and thus allowing the gas to leak from the shock.
There are also disadvantages with respect to those methods which involve axially displacing a seal against the resistance of a retaining spring. In this regard, this method has the disadvantage of requiring the use of a complex seal assembly as well as the retaining spring.